The invention relates to a process for injection blow molding an article, in which a composition, containing at least one ethylene-based polymer and little or no mold release agent, is injection blow molded.
Injection blow molding (IBM) is a process typically used to make bottles for pharmaceutical and personal care packaging, which require threaded sections with precise dimensions. The IBM process consists of three main steps: injection, blowing and ejection. During the blowing stage, sections of the pre-form may remain adhered to the core rod resulting in non-inflated or partially inflated parts. Thus, mold release agents, such as erucamide or zinc stearate (ZnSt), are commonly compounded with the polymer during polymer production, or added as “salt and pepper” blends at the time of injection blow molding, in order to prevent the polymer from sticking to the core rod. Thus, these mold release agents facilitate the release of the molded article from the mold cavity. The mold release agents typically migrate to the polymer surface, where they provide a medium between the polymer matrix and the core rod/mold surface. This migration can be problematic, because the mold release agents can result in the contamination of the contents placed inside the molded article. Typically, polyethylene resins extruded without mold release agents require more restrictive extrusion conditions. Generally, lower melt temperatures, cooler blowing conditions, and longer cycle times are required to produce an acceptable part. These restrictive conditions can result in lower quality parts and increased production costs.
Extrusion blow molding (EBM), injection stretch blow molding (ISBM), and injection blow molding are the primary blow molding processes used to fabricate plastic containers. While EBM is the most prevalent process to produce polyethylene bottles, IBM comes into play when containers with consistent weight, volume and greater detail in the neck (threaded) area need to be fabricated. Other IBM benefits include more efficient use of resin, as there is no de-flashing, trimming, re-granulation or re-mixing of scrap resin. However, there is a need for an injection blow molding process that uses a composition that requires none or minimal mold release agent. There is a further need for an injection blow molded process that produces articles of various geometries and sizes, in a wide mold/core rod temperature range.
International Publication No. WO 2008/012319 discloses the manufacture of polyethylene based wall articles with improved color and processing conditions by the use of a specific combination of stabilizers. A significant amount of zinc stearate (3000 ppm) is used in some of the formulations as a demolding or release agent.
U.S. Publication No. 2006/0155058 discloses a polyethylene composition with multi-modal molecular mass distribution, which is suitable for blow molding of small containers. The composition has a density in the range of from 0.955 to 0.960 g/cm3, and an MFR190/5 in the range from 0.8 to 1.6 dg/min. It comprises from 45 to 55 percent by weight of a low-molecular-mass ethylene homopolymer A, from 20 to 35 percent by weight of a high-molecular-mass copolymer B made from ethylene and from another 1-olefin having from 4 to 8 carbon atoms, and from 20 to 30% by weight of an ultrahigh-molecular-mass ethylene copolymer C. Calcium stearate (0.1 wt %) is used in an additive package.
U.S. Publication No. 2007/0129496 discloses multimodal polyethylenes having a density from 0.940 to 0.965 g/cm3, and an I21 from 4 to 20 dg/min, and comprising a low molecular weight ethylene copolymer having a weight average molecular weight from 5,000 amu to 50,000 amu, and a high molecular weight ethylene copolymer having a weight average molecular weight from 60,000 amu to 800,000 amu. Both components having a desirable balance of short chain branching, making the multimodal polyethylene suitable for films, pipes, rotomolding applications and blow molding applications. Zinc stearate (1000-1500 ppm) is used in an additive package of some granular resins.
U.S. Pat. No. 5,643,646 discloses a tinted bottle formed by injection blow molding a high density polyethylene. The bottles are injection blow-molded from a mixture containing a high gloss, high density polyethylene resin, a slip agent such as zinc stearate or calcium stearate, and a colorant. The mixture preferably contains about 2-6 percent by weight colorant and about 1-3 percent slip agent with the balance being high density polyethylene resin.
U.S. Publication No. 2005/0037166 discloses an injection blow molded, single layer, hollow packaging, comprising essentially a metallocene-produced polyethylene. The hollow packaging has an external and internal gloss of at least 30, and the metallocene-produced polyethylene has a density of from 0.910 up to 0.966 g/cm3 or up to homopolymer densities, and a melt index MI2 of from 0.5 to 2.5 g/10 min.
U.S. Pat. No. 7,081,285 discloses blown films and blow molded objects, prepared using a polyethylene composition comprising a unimodal metallocene catalyzed polyethylene copolymer. The polyethylene copolymer is disclosed as having a high melt flow rate and good melt strength, allowing it to be useful in both extrusion and injection blow molding.
Additional polyethylene compositions and/or molding processes are disclosed in the following: U.S. Pat. Nos. 5,747,594; 5,773,106; 6,787,608; 6,077,922; 6,822,051; 5,408,015; 6,713,561; 7,250,473; U.S. Publication Nos. 2007/0021567; 2005/0222338; 2008/0051538; 2008/0114131; 2006/0281867; 2008/0015318; 2003/0113496; 2005/0261435; International Publication Nos. WO 2007/003530; WO 2006/138084; WO 2008/008829; WO 2006/036348; WO 2008/016815; WO 2007/130515; WO 2008/051824; European Patent Application No. EP1655333A1; European Patent No. EP1040158B1; and Japanese Patent Application JP3092327A (abstract).
As discussed, there remains a need for an injection blow molding process that uses a composition that does not require the use of a mold release agent, or requires minimal use of a mold release agent. There is a further need for an injection blow molded process that can produce articles of various geometries and sizes with a wide mold/core rod temperature range. In addition, there is a need to produce such articles by reducing cycle time, increasing production rate, decreasing set times, and increasing the temperature at which injection molded parts are produced. Injection blow molded articles with a good balance of properties, such as, stiffness; environmental stress crack resistance (ESCR), and water vapor transmission barrier are also desired. These needs and others have been met by the following invention.